1. Field of the Invention
This invention is related in general to the field of interferometry and, in particular, to a novel scanning approach based on lateral translation of the sample stage or interferometric objective.
2. Description of the Related Art
Accurate profilometry is a very important component of modern technology. As the art moves towards smaller parts and more accurate manufacturing methods, there is an increased requirement for fast and reliable methods of shape measurement, quality control and process monitoring. Advances in the semiconductor and data storage industries and new technologies, such as MEMS (micro-electro mechanical systems), require measurement resolution in the order of nanometers and ever faster processing.
Among the various measurement methods, optical techniques are preferred because of their non-contact nature, their high accuracy and the three-dimensional surface information they produce. Several optical phenomena have been exploited for this purpose. Depending on the shape, size and material of the test object, these techniques preferentially use structured light, focusing properties of optics, interference of light, etc., to achieve the best possible results in an economical and practical way. Moire"" techniques, ESPI (electronic speckle-pattern interferometry), laser scanning, photogrammetry, and interferometry are only few of the many techniques developed for conducting three-dimensional shape measurements. The accuracy of these methods depends on many factors and varies from the sub-nanometer to the millimeter range. For large objects, such as car bodies, mechanical parts, etc., fringe projection, photogrammetry, or ESPI systems are usually utilized. Smaller objects, such as magnetic heads, require better absolute accuracy and methods such as interferometry or confocal microscopy are normally used.
Among these methods white-light vertical scanning interferometry (VSI), also commonly referred to as white-light interferometry or coherence radar, is used for small objects with roughness that does not exceed a few micrometers. The method is based on detection of the coherence peak created by two interfering, polychromatic wavefronts. It has many advantages such as absolute depth discrimination, fast measurement cycle, and high vertical resolution. It has proven to be very effective in measurements of objects with surface dimensions in the range of 15 mm down to sub-millimeter sizes.
Since its early development by Flourney at al. (P. A. Flourney, R. W. McClure and G. Wyntjes, xe2x80x9cWhite-light Interferometric Thickness Gauge,xe2x80x9d Appl. Opt. 11, 1907-1915, 972), the procedure has been significantly improved and commercially available systems have measurement repeatability of 0.1 nm or better. One of the important advantages of VSI is the ease with which it can be combined with other measurement techniques, such as phase-shifting interferometry (PSI), which are superior in accuracy but lack the scanning depth of VSI. For example, both techniques were combined in an system described as xe2x80x9cPSI on the flyxe2x80x9d (see U.S. Pat. No. 5,133,601 to Cohen et al.), and in U.S. Pat. No. 5,471,303 to Ai et al., wherein information from PSI is used to refine the height resolution of the VSI technique.
As is well understood in the art of vertical-scanning and phase-shifting interferometry, the optical path difference (OPD) between a test beam and a reference beam is varied in order to make a measurement. This is typically accomplished by shifting either the test surface or the reference surface of the interferometer along the optical axis by a predetermined distance during or between times of acquisition of data frames. The shift is normally carried out in steps or by continuous motion at a known, ideally constant, speed.
In the stepping method, the sample surface (or, alternatively, the reference surface) is moved between data frames and held still during data acquisition; thus, the OPD is kept constant during acquisition of each data frame. In practice, the shift-and-hold motion of the stepping method is mechanically undesirable because a finite amount of time is required for the shifted portion of the apparatus to settle into a static condition, thereby slowing down the process of data acquisition. Therefore, this method is no longer generally preferred in the industry.
In the ramping method, the OPD is varied in a continuous, smooth fashion, typically by scanning either the sample surface or the reference surface at constant speed throughout the measurement sequence. This approach is more common for vertical scanning interferometry and phase shifting because it allows faster measurements than the stepping method.
A typical vertical scanning interferometer 10 is shown in FIG. 1. A test sample or object 12 is placed in an interferometric setup illuminated with a xe2x80x9cwhitexe2x80x9d light source, such as an incandescent light bulb or LED 14. The beam from the source, which may be passed through a broadband filter (not shown), is collimated in collimator optics 16 and divided into object and reference beams OB and RB, respectively, using a beam splitter 18. The light reflected from the surface S of the object and from a reference surface 20 is then combined and the resulting image is projected on a CCD camera 22 through appropriate imaging optics 24 for registration and further processing by a computer (not shown).
The contrast of registered fringes, resulting from the interference between the object and reference beams, is proportional to the modulus of the complex degree of the mutual coherence of the wavefronts OB and RB. Since the light source is polychromatic, it has a low degree of time coherence and, therefore, the interference can occur only in a limited space around the coherence plane P defined by a zero optical path difference (OPD).
This property is used for retrieval of the object""s shape. The object is scanned along the optical axis of the instrument (referred to as the z direction in the figure) such that different heights pass through the coherence plane. In this limited coherence space the interference fringes become visible as the modulation of intensity along the scanning path, the peak of the fringe contrast corresponding to a vertical position where the OPD is zero. During the scan a number of intensity frames is acquired at defined locations and the intensity from each detector pixel (corresponding to a single point on the object) is analyzed as a function of the object""s vertical position. Normally the maximum of the intensity modulation envelope, or its center, defines the relative height of the observed object point. The profile of the correlogram is defined by the equation:
I(x,y,z)=a(x,y)+m(x,y)c[zxe2x88x922h(x,y)]cos [2xcfx80wozxe2x88x92xcex1(a,y)]xe2x80x83xe2x80x83(1)
where I(x,y,z) is the intensity at location (x,y) and height z on the sample surface; a(x,y) is the average intensity; m(x,y) is the modulation; c(z) is the envelope function defined by the spectral properties of the light source; h(x,y) is the height of the object; wo is the mean wavelength of the source; and xcex1(x,y) is the initial phase difference between the object and reference beams. See G. S. Kino and S. S. C. Chim, xe2x80x9cMirau correlation microscope,xe2x80x9d Appl. Opt. 29, 3775-3783 (1990).
A typical correlogram C from a single pixel of an object obtained by the vertical scanning method is shown in FIG. 2. The high frequency modulating signal arises from the interference of light while the envelope corresponds to the contrast of fringes, i.e., the modulus of the mutual degree of coherence of the interfering wavefronts.
Subsequent data processing is used to retrieve the accurate position of the peak of the envelope. Many algorithms have been devised in the art to detect the location of the maximum. In most cases, the intensity signal detected at the camera 22 is differentiated to remove the DC term and subsequently a center of mass, Fourier analysis, or curve fitting approach is used to perform this task.
Usually the scanning speed of the interferometer 10 is synchronized with the carrier frequency, i.e., the modulation of the intensity signal. Ninety-degree, 120xc2x0 or 60xc2x0 intervals are preferred because of their immediate application to other techniques, such as phase methods. Faster scanning speeds (such as every 450xc2x0) are also used, but with reduced performance. The total scanning speed is limited by the camera frame rate acquisition and the scanning step size. Typical scanning speeds for commercial products do not exceed 15 xcexcm/sec.
VSI systems are commonly built on the basis of a microscope; therefore, the field of view is limited to less than about 15 mm. When a larger part needs to be measured, the total area is split into a number of overlapping regions that are measured independently. Subsequently, a stitching technique is implemented to reconstruct the entire surface. (See, for example, U.S. Pat. No. 5,987,189 and U.S. Pat. No. 5,991,461). The disadvantage of this approach is that some areas are measured more than once because overlapping regions are needed for the stitching routines to match adjacent measurements. This requires extra time. In addition, measurement inaccuracies introduce errors into the procedure of stitching the sub-measurements. The approach also carries a risk of producing erroneous waviness in the stitched data set when the mean shape of the object deviates from flat. In such a case the scanning range must also be adjusted to accommodate the total bow of the surface.
In view of these shortcomings of the prior art, it would be very desirable to provide an interferometric profiling approach with the flexibility of operating within an expanded range of sample surface without loss of precision or resolution. This invention provides a novel solution to that end.
One primary goal of this invention is a novel approach to scanning interferometry, especially broadband VSI, that permits the direct measurement of an expanded region of a sample surface without stitching of multiple data sets.
Another important goal of the invention is a broad bandwidth interferometer suitable for direct measurement of an expanded region of the sample surface using a conventional microscope objective and VSI or PSI hardware.
Another goal is a solution that provides substantially uniform and continuous scanning of a sample-surface region of interest.
Still another objective is an approach that is suitable for implementation with all prior-art interferometric arrangements.
Finally, a goal of the invention is the development of a method and apparatus that are suitable for implementation by modifying existing interferometric surface profilers.
Therefore, according to these and other objectives, the invention consists of scanning the sample surface laterally with respect to the optical axis of the interferometric objective. The optical axis of the instrument is tilted, so that the sample surface is placed at an angle with respect to the maximum coherence plane of the instrument. By moving the sample stage or the objective laterally, at such an angle, so that the stage passes through a point at a set distance from the objective on the objective""s optical axis, rather than vertically along the optical axis, different parts of the object intersect the maximum coherence plane at different times as the surface passes through the coherence plane, the precise time depending on the profile of the surface. When the OPD of a point on the object""s surface is greater than the coherence length of the light source, the intensity of light reflected from this point does not produce interference fringes. Therefore, the intensity registered by the detector is approximately constant. However, when the object point enters the zone of coherence, the interference effects modulate the intensity the same way as in a regular VSI procedure. As the object moves along the scanning direction, it also has a relative vertical speed with respect to the objective because of the tilt of the objective""s optical axis with respect to the scanning plane; therefore, the lateral scanning motion produces an OPD variation as the vertical scan in a conventional system. As a result of the novel scanning approach of the invention, light intensity data are acquired continuously as the test surface is scanned, thus elimination the need for stitching multiple sets when elongated objects are tested.
Various other purposes and advantages of the invention will become clear from its description in the specification that follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiment and particularly pointed out in the claims. However, such drawings and description disclose but one of the various ways in which the invention may be practiced.